After the collapse of the Soviet Union, many worried that the republics wouldn’t be able to keep control over the weapons they had inherited.

The cold war--and with it the Soviet Union--are now history. Still with us, however, are enormous nuclear arsenals on both sides. After the collapse of the Soviet Union, many worried that the republics wouldn’t be able to keep control over the weapons they had inherited. Posing a particular risk were the tactical nuclear warheads, many of which are so small and light that they can be carted around in a station wagon.

Fortunately, this danger has led to plans to store or dismantle warheads on an unprecedented scale. Last September President Bush announced that the United States would scrap more than 3,000 tactical nuclear weapons. These include nuclear artillery shells, nuclear warheads for short-range Lance missiles, and nuclear antisubmarine bombs. He also ordered all the tactical weapons on ships and subs to be put in storage. In making this move Bush hoped that former Soviet president Gorbachev would make reciprocal cuts, and indeed, a week later Gorbachev did so, announcing that all Soviet tactical nuclear weapons would be either dismantled or stored.

Soon the staggering scale of the Soviet disarmament became clear. In a meeting in Washington ten days after Gorbachev’s announcement, Victor Mikhailov, the deputy minister responsible for nuclear warhead production, estimated that between 10,000 and 20,000 Soviet warheads would be taken apart. The republics are cooperating, and tactical warheads are being taken to Russian dismantling sites. In addition, other agreements should lead in the coming years to thousands of warheads for long-range strategic missiles and bombers being dismantled.

Although politicians can order the elimination of nuclear bombs with the stroke of a pen, actually taking them apart isn’t so easy. The sheer number guarantees that it will take several years to dismantle them all. The United States has had a certain amount of practice, since it has cannibalized the plutonium in old warheads to put in new ones for decades. But taking apart a missile so that the materials can never be used again is a new challenge. Fortunately, the Federation of American Scientists has been working with Russian scientists on the problem for the past five years in a project that I direct.

Before explaining how to take a nuclear bomb apart, it’s necessary to explain how one works. The explosive power, or yield, of a warhead is usually measured by how many thousands of tons (kilotons) of chemical explosives would be needed to create the same blast. Most tactical nuclear weapons, designed for short-range use on the battlefield instead of intercontinental attack, have relatively low yields, in the range of .1 to 10 kilotons. Some kinds of tactical weapons, however, and long-range strategic missiles and bombers carry more powerful warheads with yields as high as several hundred kilotons. In comparison, the bombs that destroyed Hiroshima and Nagasaki had yields of around 20 kilotons.

A modern warhead in the kiloton range is usually just a miniaturized version of the Nagasaki warhead--a bomb that creates energy by splitting plutonium atoms. Chemical explosives surround the core, and when they are set off they crush it until its density roughly doubles. At the same time, a tiny particle accelerator in the warhead sprays neutrons into the plutonium. When the neutrons hit the plutonium atoms, they spontaneously split apart--a process known as fission--into smaller atoms and free neutrons. These neutrons run into other atoms, split them apart and release more neutrons, and so on. Each fissioning plutonium atom releases two or three neutrons, so that with each collision the fission rate more than doubles. It happens so fast that in less than a millionth of a second one fissioning atom can cause 3 billion trillion other atoms to do the same. The energy released as this happens gives 2 pounds of plutonium the same explosive power as 15,000 tons of TNT.

Modern fission bombs get an extra kick from a small supply of heavy hydrogen isotopes (different isotopes have different numbers of neutrons in their nuclei). A normal hydrogen atom has in its nucleus only a single proton, but deuterium has a neutron also, and tritium carries two. As the plutonium fissions and heats to more than 100 million degrees, the hydrogen isotopes are injected from a canister into the bomb’s core. At that temperature the isotopes start fusing, forming helium and free neutrons. The extra burst of neutrons speeds up the fissioning, allowing it to go further before the plutonium blows itself apart. This makes it possible to build lighter bombs with less plutonium.

Warheads with yields in the hundreds of kilotons make fusion the centerpiece of the explosion. For a trigger they use a fission bomb, which creates the pressure and heat necessary for a second, more powerful chain reaction to take place. Neutrons strike lithium atoms in a lithium- deuterium compound, creating tritium and helium. The tritium then hits deuterium, generating helium and another free neutron, which can then hit a lithium atom, starting the fusion cycle anew. Bomb makers also add uranium since some of the neutrons set free by the fusion cause the uranium to fission, giving the explosion added power.

These then are the elements involved in making one of these horrendous weapons. What does it take to unmake it? The first step is simply to disable it so it can’t work. That requires removing the tritium canister and the wiring that delivers the precisely timed electric signals that set off the chemical explosives.

If left this way, though, a warhead can easily be made operational again. Someone can simply put in new circuitry and hook on a new canister. It’s important to take measures to prevent such a quick fix. At a recent meeting of our group in Kiev, one U.S. weapons expert came up with a very simple idea: Pour molasses into the tritium injection hole. You know how when you try to get that last bit of molasses out of a bottle, it just won’t come out? When the plutonium implodes, it must be crushed into a perfect sphere to become dense enough for a chain reaction to take place. If some substance--molasses or otherwise--gets into the core, it makes this perfect implosion of pure plutonium impossible.

Even when a warhead is nuclear safe, however, it can still kill with its chemical explosives, which are just modern relatives of dynamite. Not only can they create a blast as intense as a small conventional bomb, but they can also throw up a cancer-causing cloud of plutonium. Steve Fetter of the University of Maryland and I have done calculations showing that if such an accident happened at the ballistic-missile submarine base in Bangor, Washington, and a plutonium dust cloud was blown 20 miles to Seattle, as many as a thousand people inhaling the cloud could die of cancer in the following decades.

To avoid this risk bomb makers have developed insensitive explosives. These can’t be detonated accidentally or intentionally by airplane crashes, fires, or gunshots. Unfortunately, many American warheads waiting to be dismantled (and probably Soviet ones as well) were built before insensitive explosives were developed. The risks are minimized by removing the explosives one warhead at a time in heavily protected bunkers.

Once the warheads are stripped of chemical explosives, the next step is to deal with the uranium and plutonium inside. If a country or terrorist organization stole 10 pounds of plutonium or 40 pounds of highly enriched uranium, it could make a simple fission bomb as powerful as the one dropped on Nagasaki. Considering how unstable conditions are now in the former Soviet Union, the danger of such nuclear theft may be quite real. It would therefore be in the interests of the United States and the world if the dismantled warheads and their plutonium and uranium were put under some kind of bilateral or international safeguards.

There is a catch, however. Russian officials say that they will agree to put their recovered plutonium and uranium and their production facilities under this kind of protection only if the United States does the same. Some members of the Bush administration don’t want to take this step, because we’ll be giving up the ability to recycle these bomb materials. Paul Wolfowitz, the Undersecretary of Defense for Policy, has gone so far as to argue that the United States should not just keep all the materials recovered from the dismantled warheads for possible reuse but also maintain the option of producing new nuclear weapons material.

My own view is that the United States should agree to seize the opportunity to lock in reductions in both nuclear arsenals. After all, the entire rationale for the U.S. nuclear stockpile has been the huge Soviet one. Now that they are going to reduce, we can reduce as well.

As each side takes apart its bombs, it will have to be able to verify that the other is playing by the rules. The Federation of American Scientists and Russian scientists have cooperated to work out the necessary arrangements to ensure that the weapons are dismantled and that their materials aren’t used in new ones. The warheads would be put into containers that inspectors would seal and tag. The inspectors would check the seals and tags periodically. When they reached a dismantlement facility inspectors would also measure the amount of gamma rays and neutrons coming out of the container to check the kind of warhead inside. The inspectors would also monitor the perimeter of the facility with various instruments, making sure that uranium and plutonium leaving the grounds would be immediately placed under international safeguards--possibly those of the International Atomic Energy Agency--and not spirited out into unknown hands.

At this point the only task left would be to dispose of the material. The uranium wouldn’t be hard to deal with. The uranium carried in most nuclear warheads is highly enriched, often containing more than 90 percent of the chain-reacting isotope U-235. Natural uranium ore is made up of only .7 percent U-235; the rest of it is U-238, which carries three more neutrons. By mixing the highly enriched uranium from dismantled warheads with natural uranium, workers could dilute it down to the 3 to 4 percent level in power reactors.

Once the uranium is diluted, it can no longer produce an explosive chain reaction. If someone wanted to use this stuff to get concentrated U-235 again, he would need to get his hands on an isotope- separation plant. There are only six of these expensive facilities in the United States and Russia (we have two, they have four). Our group is proposing that they be put under international safeguards.

The uranium could well become the big payback of dismantlement. Even with low-enriched uranium prices at an all-time low, the 25 to 50 pounds of U-235 in an average warhead would be worth anywhere from $170,000 to $340,000 as nuclear reactor fuel. Fifteen thousand dismantled warheads would generate about $4 billion. This would more than refund the cost of dismantlement, and so it should be an incentive to everyone involved to get on with the job. By cutting nuclear arsenals on both sides by half, we would free up roughly one million pounds of U-235, which could fuel the world’s nuclear power plants for a year and a half.

At first blush it would seem that we could do the same thing with plutonium. Experimental reactor fuel spiked with several percent plutonium has been successfully tested in ordinary nuclear power plants for decades. Dismantling half the arsenals would make available more than 200,000 pounds of plutonium; with that you could run all the nuclear reactors in the world for three months.

Experience has shown, however, that using plutonium would be expensive and dangerous. The protections needed to handle this lethal material would make it twice as expensive as regular uranium fuel. Even worse, no plutonium isotope exists that could be used to dilute the plutonium removed from bombs to make a chain reaction impossible. Even if it’s mixed in small amounts into uranium reactor fuel, someone could steal the fuel, extract the plutonium chemically, and have the raw ingredient for a nuclear bomb. As a result, fresh nuclear fuel containing plutonium would have to be guarded almost as tightly as nuclear weapons are now.

If it continues to make no sense to use this plutonium as nuclear fuel, it will have to be disposed of as radioactive waste. That brings up another problem: the nuclear power establishment hasn’t been able to settle on a way to get rid of radioactive waste that the public can wholeheartedly accept. The industry is currently putting almost all its efforts into burying waste 1,000 to 2,000 feet underground. Since radioactive waste stays hazardous for centuries (the main isotope of plutonium in warheads has a half-life of 24,000 years), it’s been impossible to prove that none of it will make its way back to the surface, whether carried up by groundwater or some other means. With local opposition flaring up just about everywhere a radioactive burial site is proposed, some researchers have been thinking about more exotic and more expensive ways to dispose of radioactive material.

One plan goes by the name transmutation. For plutonium this is just another name for fission. If plutonium is mixed with other materials and then bombarded with neutrons, the atoms split into smaller ones with shorter half-lives without going through an explosive chain reaction. Conventional nuclear reactors produce neutrons, but they have so little energy that often, instead of breaking a plutonium nucleus apart, they stick to it. There are prototypes of a different kind of plant, known as a fast neutron reactor, that could produce neutrons energetic enough to fission the plutonium more effectively. The problem is that they will probably cost twice as much as current reactors.

The same method could also be used with a twist, employing a particle accelerator instead of a reactor. The accelerator could generate a beam of protons that would slam into a target such as lead. The impact of the protons would generate a shower of fast neutrons that could break down the plutonium. Again the problem is cost.

A third idea--even more exotic and probably at least as expensive--is to shoot the plutonium into the sun. Nuclear expert Theodore Taylor, who has been examining this approach, has pointed out that it would be within our technological grasp. Even with radiation shielding and neutron-absorbing material to prevent a chain reaction, the load would not be too great to lift into space.

According to Taylor’s scheme, heavy booster rockets would be used, such as the Saturn V--used for the Apollo moon missions--or the new Soviet Energia. The rockets would bring packages containing a few tons of plutonium each into a high-altitude orbit around Earth. A solar-powered space tug would gradually push the plutonium into solar orbit and then slow it down over a period of weeks. The plutonium’s orbit would decay until it fell into the sun, a nuclear reactor par excellence that would destroy the waste without a trace.

One obvious question with this scheme is what happens if the boosters fail? According to Taylor, the load could be designed to survive such dire effects as the explosion of the rocket or the heat experienced upon reentry into Earth’s atmosphere or even the impact when it hit the ground. For now, though, those are just Taylor’s claims; his calculations have yet to be reviewed by other experts and published.

It’s worth keeping in mind that most of the world’s plutonium isn’t in nuclear warheads but in spent reactor fuel. (It’s created when atoms of U-238 absorb neutrons.) If we’re going to go to all the trouble of sending warhead plutonium into space or fissioning it with neutrons instead of burying it, it would make sense to get rid of the rest of the plutonium on our hands in the same way. That would mean making a major commitment to reprocessing the fuel to get the plutonium out, which involves dissolving the spent fuel into liquids. Sadly, this has led to severe contamination of soil and groundwater, such as at the first-generation military reprocessing plants in Hanford, Washington, and near the Russian city of Chelyabinsk in the Urals. Modern reprocessing plants are much less polluting, but they still create waste that is proving difficult to manage. As we explore the various complications of the alternatives, deep burial may, after all, look like the best choice.

With all these uncertainties and possible dangers facing us, my vote for now is to store both the plutonium from nuclear warheads and spent reactor fuel in a safe, secure place under international supervision. We are going to have to hash out the advantages and disadvantages of the different ways to get rid of this material, and the discussion will probably take years. It’s already taken us more than 40 years to begin cutting the nuclear arsenal deeply. I hope it won’t take that long to finally decide what to do with the plutonium.